Method for producing sheet-form electrolytic copper from halide solution

ABSTRACT

In a method for producing dense sheet-form electrolytic copper by electrowinning, the electrolyte contains a polyethylene glycol additive as a smoothening agent. In the prior art method, stirring is necessary. In an electrowinning method without stirring, the current concentrates on the edge portion of cathode  2 . This current concentration is mitigated by means of conducting the electrolyzing current through a window  6   b  of a shielding plate  6  located in the vicinity of a cathode  2.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to production of metallic copper by electrowinning, more particularly to a method for producing sheet-form, dense electrolytic copper from halide solution.

2. Description of Related Art

The electrowinning method is broadly implemented commercially for producing metallic copper. SX-EW (solvent extraction—electrowinning method) is a representative method implemented at present for producing metallic copper mainly from oxide ores. The copper solution yielded by the sulfuric acid leaching is purified by various processes, such as the solvent extraction and cleaning. The resultant refined and concentrated, sulfuric acid-based copper electrolyte is subjected to the electrowinning. Instead of the sulfuric acid leach liquor, a halide leach liquor such as a chloride bath has been developed for electrowinning of copper (c.f. Japanese Patent No. 2857930).

The halide leaching is advantageous in the following points.

(1) The elementary chlorine or the like is formed by the anodic oxidation of the electrowinning and is strong oxidizing agent. Sulfide ores, which are less reactive than the oxide ores, can be leached by utilizing elementary chlorine or bromine or its compounds.

(2) The leach liquor, which contains halides at a high concentration, enables stable dissolution of mono-valent copper ions. The electrolyzing of the mono-valent copper ions can, therefore, be realized. The quantity of electricity required for the electrolysis of the mono-valent copper is as low as a half that of the divalent copper-ions dissolved in the sulfuric acid bath.

(3) The ionic conductivity and exchange current density are high. The current density, therefore, does not decrease seriously at high current density and hence the productivity is high.

However, in the case of electrowinning from the halide solution, the metallic copper electrolytically deposits in the form of a dendritic powder or aggregated coarse grains. In the case of the SX-EW method using the sulfuric-acid bath, the copper electrolytically deposits in the form of a sheet. Such a sheet can be separated from the cathode as cathodically deposited copper and sold as it is.

The metallic copper deposited from the halide solution involves complicated handling processes, such as in withdrawal from the electrolytic cell, washing, casting in the form of a commercial product, and the like. In addition, even if the washing is repeated, since the copper powder is liable to be oxidized, the properties of a product deteriorate in quality.

In order to eliminate the drawbacks involved in the electrowinning from the chloride bath, experiments in a laboratory have been carried out to discover the conditions needed to obtain a smooth electrolytic product in the electrolyzing of the mono-valent ions. However, the electrolytic deposits from the halide bath, such as the chloride bath, have a strong tendency to grow dendritically and to form a number of nodules, as compared with the case of electrolytic deposition from the sulfuric acid bath. Although such additives as glue and gelatin are effective for refining and smoothening the electrolytically deposited product in the sulfuric acid bath, in order to attain similar effects in the case of operation of the halide bath, the additive content must be increased to as high as a few g/L which lowers the current density. Practical conditions are unknown for extended operation of dense and smooth electrolytically deposited copper.

The known methods necessitate vigorously stirring the catholite by means of blowing inert gas, such as nitrogen or argon, in order to obtain dense electrolytic deposit. Alternatively, the electrolyte in the vicinity of the cathode is forced to move by means of vibrating or swaying the cathode, so as to form the feeding of ions. It is, therefore, necessary to provide an electrolyzing apparatus with complicated mechanism to realize the gas blowing, the vibration and the like.

Australian patent application No. 2005202863 filed by the present applicant (hereinafter referred to as the Australian patent application) discloses a method for producing dense sheet-form electrolytic copper, wherein the electrowinning of metallic copper from the halide copper-electrolyte is carried out in the halide electrolyte, which contains a polyethylene glycol additive as a smoothening agent, and which is stirred in the vicinity of the cathode. The polyethylene glycol additive among the other additives is specifically effective for suppressing the dendrite formation and densifying the electrolytically deposited copper. In the Australian patent application, it is stated as follows. “The catholyte must be stirred by means of, for example, a stirring plate . . . so as to produce the dense electrolytic copper. When no stirring is carried out, the concentration gradient is formed in the catholyte and seriously impairs the effects of the additive, that is, densification and smoothening of the electrolytic copper. The effects of the additive are attributable to strong adhesion of the polyethylene glycol on the surface of the electrolytic copper.”

SUMMARY OF INVENTION

It is, therefore an object of the present invention to provide an electrowinning method of metallic copper from a halide solution, which is advantageously performed by electrolysis of mono-valent ions thus saving electric power, and which produces dense and sheet-form electrolytic copper having improved washing property and handling property, such as in withdrawal from an electrolytic cell. It is a specific object of the present invention to eliminate the necessity of stirring of the electrolyte proposed in the Australian patent application, while obtaining the dense, sheet-form electrolytic copper.

The present invention provides the following methods.

(1) A method for producing dense sheet-form electrolytic copper, wherein the electrowinning is carried out in the halide electrolyte being fed into an electrolytic cell, which electrolyte contains a polyethylene glycol additive as a smoothening agent, characterized in that the electrolyzing current is conducted through a window of a shielding plate located in the vicinity of a cathode, thereby mitigating current concentration on the edge of the cathode by the plate portion defining the window and having substantially the same dimension as the edge region of the cathode

(2) A method for producing dense sheet-form electrolytic copper according to (1), above, wherein the copper-halide electrolyte contains not less than 3 mol/L of alkali metal chloride, alkali metal bromide or a mixture of the alkali metal chloride and bromide as a bearing salt, and also dissolves copper in the form of at least one of chloride and bromide.

(3) A method for producing dense sheet-form electrolytic copper according to (1) or (2), above, wherein the concentration of the polyethylene glycol additive is from 20 mg/L to 40 mg/L, and further the current density is not more than 125 A/m².

The advantages obtained by the present invention are as follows.

(1) Since the electrolyte is not stirred by mechanical means, gas blowing and the like, the investment cost and running cost are low.

(2) Since the electrolytic copper deposited from the halide solution has dense structure, the electrolytic copper in the form of a sheet can be easily separated from the cathode mother plate and easily withdrawn from an electrolytic cell.

(3) The electrolytic deposit can be satisfactorily washed to eliminate contamination, while preventing the contamination or oxidation on the surface. The qualities of the product are, therefore, improved.

(4) Homogeneous electrolytic deposit can be stably obtained ever for a long period of 24 hours or longer.

Preferred embodiments of the present invention are hereinafter described with reference to the drawings, in which a predetermined amount of polyethylene glycol is added into a copper-halide electrolyte

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an electrowinning method of copper according to the present invention.

FIG. 2 is a front view of the shielding plate according to an embodiment of the present invention.

FIG. 3 illustrates a location of a shielding plate and a cathode.

FIG. 4 shows an electrolytic deposition state in Example 1.

FIG. 5 shows an electrolytic deposition state in Comparative Example 1.

FIG. 6 shows another electrolytic deposition state in Comparative Example 1, in which the polyethylene glycol additive is not added.

FIG. 7 shows an electrolytic deposition state in Comparative Example 2, in which the additive polyethylene glycol is used but the shielding plate is not used.

FIG. 8 shows an electrolytic deposition state in Example 2.

FIG. 9 shows another electrolytic deposition state in Example 2.

FIG. 10 shows a partial structure of an electrolytic cell used in Example 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The electrolyte used for electrowinning of copper from the halide solution is acidic, particularly pH 1˜3, to enable copper leaching and to prevent precipitation of the basic salt and mono-valent copper halides. High concentration of halogenated alkali, such as sodium chloride, as the bearing salt, enables the stable dissolution of the mono-valent copper halogen-complex in the leached copper-liquor. In order to attain satisfactorily high solubility of copper complex ions, the concentration of the bearing salt should be 3 mol/L or more, preferably 4 mol/L or more, and less than the saturation solubility.

The electrolysis of the mono-valent copper is carried out by using a diaphragm electrolysis method, in which the anode and cathode chambers 10 and 11, respectively, are separated by a filter cloth 4. Referring to FIG. 1, 1 denotes an electrolytic cell, 3 denotes an anode, and 2 denotes a cathode. The liquor directly after the ore leaching contains an appreciable amount of divalent copper ions, because the copper ions in such liquor may be oxidized due to air oxidation or the leaching reactions may be insufficient. The divalent copper ions incur undesired consumption of cathode current, which upsets the balance between the cathode and anode reactions during the electrolysis. The liquor directly after the ore leaching is, therefore, preliminarily transferred to a reducing tank, and the metallic copper added in the reducing tank reduces the divalent copper ions to mono-valent copper ions. The so-treated liquor is fed to the cathode chamber of the electrolytic cell 1. The polyethylene glycol additive is preliminarily mixed with the fed electrolyte and is then fed into the cathode chamber 11. Alternatively, the polyethylene glycol additive may be directly fed into the cathode chamber 11. The additive is stirred and mixed with the catholite in the cathode chamber 11.

The polyethylene glycol additive used has an average molecular weight, which indicates the polymerization degree, in a range of from 600 to 4,000, preferably from 1,000 to 2,000. When the molecular weight is less than 600, the effect of such a low-molecular weight polymer on densifying the electrolytic deposit is poor and is not practical. On the other hand, when the molecular weight is more than 4,000, although the electrolytically deposited product is generally dense, the deposit is liable to grow in the form of a number of nodules or unevenness around the edges of the cathode, where the current concentrates. High molecular weight polyethylene glycol is, therefore, inappropriate for continuous operation over a long period of time.

Appropriate concentration of the additive is dependent upon the electrolytic conditions, such as the cathode current density and the like, and the stirring condition of the catholite in the cathode chamber 11. When the catholite in the vicinity of a cathode is so strongly forced to flow by means of mechanical stirring that feeding of the additive to the cathode surface is promoted, dense smooth electrolytic deposition is obtained even at the additive concentration in a range of from 2 to 10 mg/L. When the electrolyte is not mechanically stirred, the mixing of the electrolyte is caused by spontaneous convection, when the electrolyte in the electrolytic cell is replenished. This is the case of the sulfuric acid bath, and dense electrolytic deposition is attained.

In the halide bath, which is not mechanically stirred, the concentration of additive is preferably as high as from 10 to 100 mg/L, preferably from 20 to 40 mg/L. When the additive concentration is less than the range mentioned above, the additive is so ineffective for densifying and smoothening the electrolytic deposits that nodules locally grow on the cathode surface and may incur short circuiting between the cathode 2 and the shielding plate 6 or the diaphragm 4. The additive concentration may be increased to a level more than the range mentioned above, but its effects are not improved greatly.

The post-electro-won liquor, in which the copper concentration has been decreased due to the electrolytic deposition, is transferred to the anode chamber 10 and is converted to the anolyte, Such liquor may be returned to the leaching process. Since the additive concentration is higher in the present invention as compared with that of the Australian patent application, the amount of undecomposed additive in the liquor, which is returned to the leaching process, may be higher than that in the Australian patent application. The polyethylene glycol additive in a range of from 20 to 40 mg/L does not practically incur a problem of the undecomposed additive, provided that the current density is maintained within a preferable range of the present invention.

The current density and the current distribution on a cathode are important factors for stably depositing the dense smooth copper. In the halide bath containing the polyethylene glycol additive, the current locally concentrates on the edge of a cathode 2 when the current density exceeds a certain upper limit. The nodules abruptly abnormally grow on the edge of a cathode 2 and disadvantageously cause the short circuiting between the cathode 2 and the anode 3 or the diaphragm 4. In the electrolyte, which is mechanically stirred, the electrolytic deposition occurs densely and hence no short circuiting occurs even at a current density as high as 250 A/m². It is preferable to keep the current density to 125 A/m² or less in the case of operation without the mechanical stirring. When the current density is 150 A/m² or more, it is difficult to prevent the abnormal growth of nodules or surface unevenness of the deposited copper, unless such measures for promoting the flowing and mixing of the electrolyte as circulation or stirring are employed.

The current may concentrate on the edge of a cathode as described above. That is, the effective current density on the edge of a cathode is high, no matter how the average current density of the cathode as a whole is low. The nodules are, therefore, likely to form on the edge. A shielding plate according to the present invention mitigates the current concentration on the edge of a cathode. The shielding plate has a window, which is defined by the plate portion of the shielding plate.

Generally, the mother plate of a cathode is square or rectangular in shape and 500˜1000 mm in width and 500˜1000 mm in height. The edge of a cathode therefore extends along the square or rectangular shape mentioned above. The width of an edge of a cathode is a region where the nodules are likely to grow, and is usually from 10˜15 mm. The edge plus the width mentioned above is referred to herein as the edge region. The surface of a cathode except for the edge region is hereinafter referred to as the majority portion. The window 6 b (FIG. 2) of the shielding plate 6 has substantially the same size as the majority portion, and the plate portion 6 a defines the window 6 b which has substantially the same size as the edge region.

The shielding plates 6 are located between the cathode 2 and the diaphragms 4 in such a manner that the window (6 b, FIG. 2) faces the majority portion of a cathode 2. Desirably, the shielding plates 6 are located as close as possible to the cathode 6. Consequently, the electrolyzing current mainly concentrates on the majority portion, and the current concentration on the edge of a cathode 2 is mitigated. Practically, the distance between the shielding plates 6 and the cathode 2 should be such that the cathode can be lifted above, when the deposited copper reaches a predetermined thickness. Usually, the cathode is lifted above at a frequency of once per 7 to 10 days of the electrolysis. During this period, the copper deposits on the mother plate to a thickness of 10 to 15 mm including thickness of unevenesses, with the result that the distance between the shielding plate and the cathode is narrowed as compared with the beginning of the electrolysis, that is, directly after the location of the shielding plate. The distance between the shielding plate and the cathode is, therefore, at least greater than the depositing thickness of copper between the adjacent lifting cycles.

During the electrolysis under the range of current density mentioned above, the electrolytically deposited copper 5 (FIG. 1) grows with the result that the distance between such copper 5 and the window of the shielding plate 5 decreases. In such a circumstance, the current conduction through the window may be disadvantageously modified, so that nodules may be formed or abnormal current conduction may takes place. In order to prevent the generation of nodules and the like, the size of the window 6 b (FIG. 2) is preferably determined taking into consideration the distance between the shielding plates 6 and the cathode 2 at beginning of the electrolyzing, that is, the size of window is at least a half as large as the distance.

Referring to FIG. 10, a position of the shielding plate 6 and the cathode 2 is illustrated. In FIG. 10, the symbol “X” denotes the distance between the shielding plate 6 and the cathode 2, while the length “Y” denotes an edge portion of the cathode 2 behind the shielding plate 6. During the electrolysis, the copper grows on the cathode and the nodules are liable to form on the edge of the cathode. Preferably, the length X is greater than half of the distance “Y”. i.e., Y>X/2, so that neither nodule formation nor abnormal electrolytic deposition can occur.

A shielding cloth of the diaphragm 4 may be modified to have a window corresponding to the window of the shielding plate as shown in FIG. 4. Even if the shielding plate 6 is omitted, the window of the diaphragm mentioned above can concentrate the electrolyzing current on the majority portion of a cathode, provided that the distance between the diaphragm 4 and the cathode 2 is extremely narrowed. The diaphragm 4 is, therefore, damaged by the cathode 2, when the cathode 2 is lifted upward, because the cloth is of low strength.

The cathode 4 is made of material, such as titanium, having satisfactory corrosion-resistance against the halide copper-electrolyte. After completing the electrolysis, the electrolytic copper 5 is peeled from the cathode 2. Alternatively, a copper mother plate is used to deposit the electrolytic copper on it as in the case of the production of electrolytic copper using the sulfuric acid bath. The electrolytic copper and the copper mother-plate are cast together in the form of the product. The appearance and structure of the electrolytic copper are virtually unaffected by the underlying material except for only during the initial period of electrolytic deposition.

In the diaphragm electrolysis according to the present invention, the copper concentration of the catholyte decreases after the electrowinning. The catholite then passes through the diaphragm 4 into the anode chamber 10, where the mono-valent copper is oxidized to divalent copper due to the anodic oxidation. The anolyte contains therefore the mono-valent copper and also contains the oxidizing chlorine and bromine as well as undecomposed parts of the polyethylene glycol added in the catholyte. The anolyte may be returned to the leaching process of copper ore, for leaching copper from the copper ore. The undecomposed and remaining polyethylene glycol is preferentially adsorbed on the leaching residue of the copper ore and therefore, is removed from the liquor. The undecomposed and remaining polyethylene glycol, therefore, does not accumulate in the liquor.

As is described hereinabove, the present invention utilizes the advantages of the mono-valent electrolysis, and hence electric-power-saving, and simultaneously produces dense electrolytic copper which has improved handling performance.

EXAMPLES Example 1

Copper sulfide ore was leached by a copper chloride solution containing 4 mol/L of the halogenated alkali. The copper leaching liquor is a cuprous chloride solution containing 75 g/L of copper and is then diluted for the electrolysis by a liquor containing the halogenated alkali to decrease the copper concentration to 25 g/L. The pH was adjusted to pH 1˜1.5. The polyethylene glycol (average molecular weight—1,000) was added to the diluted cuprous chloride solution to provide 10 mg/L of the concentration. The composition of the electrolyte and replenishing liquor fed to the electrolytic cell is shown in Table 1.

TABLE 1 Composition of Liquor in Example (g/L) Fed Liquor Electrolyte (Copper Leaching Liquor) NaCl 220-230 g/L 220-240 g/L (3.8-3.9 M/L) (3.8-4.3 M/L) NaBr 28 g/L 28 g/L Cu 25-28 g/L 72-80 g/L (as CuCl) (39-44 g/L) (113-125 g/L)

An electrolysis cell used in this example is shown in FIG. 3. The reference numerals denote the same parts of the electrolytic cell shown in FIG. 1. An electrolytic cell 1 was provided with a diaphragm 4 made of acid-resistant filter cloth (trade name Tetron). The electrolyte was admitted in the electrolytic cell 1 and was maintained at a temperature of from 50 to 55° C. A cathode 2 located in a cathode chamber 11 of the electrolytic cell 1 is a titanium plate having a 100 mm square effective surface. The surface of the cathode 2 opposite to the anode 3 was exposed, while the other surface was masked. An anode 3 located in the anode chamber 10 was an insoluble anode made of a titanium plate, on which an iridium compound was applied and baked. The effective surface of the anode 3 was 100 mm square.

A shielding plate 6 was distant from the cathode surface by 10 mm and is provided with a window 80 mm square. The window of the shielding plate 6 was positioned concentrically with the center of the cathode surface.

The halide bath in the electrolytic cell 1 was replenished with the copper leaching liquor, to which the polyethylene glycol (average molecular weight—1,000) was added at a concentration of 10 mg/L. The current conducted through the electrodes 2 and 3 was 1.23 A, that is, 123 A/m² in average of the cathode surface and 150 A/m² on the majority portion of the cathode, i.e, the portion of the cathode facing the window.

The replenishing liquor, that is, the copper leaching liquor, to which the polyethylene glycol (average molecular weight—1,000) was added at a concentration of 10 mg/L, was continuously added into the cathode chamber 11 so as to maintain the copper concentration of the catholite within a level of from 25 to 28 g/L. When the total current conduction time amounted to 88 hours, the electrodeposition was terminated. The electrolytically deposited copper is shown in FIG. 4. The electrolytically deposited copper is dense and exhibited slight unenvenesses.

Comparative Example 1

In the present comparative example, the prior art without the additive was tested. The same electrolytic cell as in Example 1 was used. The same electrolyte and the same replenishing liquor as in Example 1 were used, except that the polyethylene glycol additive was added to neither the electrolyte nor the replenishing liquor. In addition, the current conducted was 1.5 A, that is, 150 A/m² on the cathode surface in average. The current was conducted for 24 hours.

The electrolytically deposited copper is shown in FIGS. 5 and 6 and is fine aggregates of the dendrites. The electrolytically deposited copper easily fell from the cathode, when it was lifted upwards.

The present comparative example was also carried out under the current of 1 A, i.e., 100 A/m² of current density in average. The electrolytically deposited copper is shown in FIG. 5 and is somewhat dense as compared with the case of current density of 150 A/m². The electrolyzing amounting to 44 hours was possible. The electrolytically deposited copper, remained in the form of aggregates of the dendrite, as well. It turned out, therefore, that the dense electrolytically deposit copper cannot be obtained even under low current density, when the additive is not added.

Comparative Example 2

In the present example, the effects of a shielding plate were tested. Namely, the same electrolyte and the same replenishing liquor as in Example 1 with the addition of polyethylene glycol were used. However, the shielding plate was not used. The current concentrated on the edge of a cathode, nodules were formed and the copper electrolytically deposited on the edge in an abnormal manner. The same electrolytic cell as in Example 1 was used. The electrolyzing current of 1.5 A, i.e., 150 A/m², was conducted for 24 hours. The electrolytically deposited copper is shown in FIG. 7. The electrolytically deposited copper was dense at the center of the cathode. However, a number of modules were generated on the edge of the cathode. Several of the cathodes abruptly grew toward the diaphragm 4 (FIG. 3) and caused the short-circuiting between the cathode and the diaphragm.

Example 2

In this example, how stable electrolytic deposition is possible in a large-sized electrolytic cell shown in FIG. 1 was investigated The diaphragm electrolytic cell shown in FIG. 1 comprises two anode chambers, and an anode chamber sandwiched the two anode chambers. The electrodes 2, 3 were large sized. A relationship between the current density and additive concentration in the large-sized electrolytic cell was investigated.

A cathode 2 located in a cathode chamber 11 of the electrolytic cell is a titanium plate having a 300 mm square effective surface. A pair of the anodes 3 located in the anode chambers 10 beside the cathode chamber were insoluble anodes made of a titanium plate having 270 mm square effective surface. A pair of the shielding plates 6 were located on both sides of the cathode 2 and were distant from the cathode 2 by 18 mm. A window 270 mm square was formed through the shielding plate 6. The window of the shielding plate 6 was positioned concentrically with the center of the cathode surface.

The current conducted was 16 to 24 A, that is, the current density of 100˜150 A/m² of the window of the shielding plate 6. The replenishing liquor was continuously fed to the cathode chamber 11. The additive was added to the electrolyte and the replenishing liquor so that the additive concentration in them varied in a range of from 10 to 40 mg/L. Current conduction was carried out for 7˜13 days, while the copper concentration of the catholyte was kept at approximately 25 g/L. The electrolytic deposition is shown in Table 2.

TABLE 2 Additive Concentration, Current Density and Change of Electrolytically Deposited State depending upon Current Density Current Additive Density (M/L) (A/m²) Electrolytic Deposits 10 150 X Frequent short circuiting occurs within 24 hours 20 100 ◯ Stable electrolytic deposition for 13 days (308 hours) 20 125 ◯ Stable electrolytic deposition for 10 days (234 hours) 40 100 ◯ Stable electrolytic deposition for 10 days (240 hours) 40 125 Δ Stable electrolytic deposition for 7 days (188 hours). Nodules generated at 8 days. 40 150 X A number of small nodules generated at 8 hours. The electrodeposition stopped.

Dense sheet-form copper electrolytically deposited without the generation of nodules, when the additive concentration was 20˜40 mg/L and the current density was 100˜125 A/m². An example of the electrolytically deposited copper is shown in FIG. 9. The electrolytically deposited copper on the cathode is a dense sheet form although unevenness is present on the surface. The electrolytically deposited copper could be entirely peeled from the mother plate in the form of a single sheet. Contrary to this, when the current density was 150 A/m², unevenness became apparent even on the center of a cathode after a short electrolyzing time. The electrolytic deposition state was instable. As shown in FIG. 9, nodules are formed on the center of the cathode after the deposition of 24 hours. The short circuiting between the cathode and the diaphragm frequently occurred. 

1. A method for producing dense sheet-form electrolytic copper, wherein the electrowinning is carried out in the halide electrolyte being fed into an electrolytic cell, which electrolyte contains a polyethylene glycol additive as a smoothening agent, characterized in that the electrolyzing current is conducted through a window of a shielding plate located in the vicinity of a cathode, thereby mitigating current concentration on the edge of the cathode by the plate portion defining the window and having substantially the same dimension as the edge region of the cathode.
 2. A method for producing dense sheet-form electrolytic copper according to claim 1, wherein the copper-halide electrolyte contains not less than 3 mol/L of alkali metal chloride, alkali metal bromide or a mixture of the alkali metal chloride and bromide as a bearing salt, and also dissolves copper in the form of chloride or bromide.
 3. A method for producing dense sheet-form electrolytic copper according to claim 1 or 2, wherein the concentration of the polyethylene glycol additive is from 20 mg/L to 40 mg/L, and further the current density is not more than 125 A/m². 